GtkMenuTrackerItem: add an internal 'visible' flag

GtkMenuTrackerItem: add an internal 'visible' flag

Add an internal API for checking if a GtkMenuTrackerItem is visible,
along with a signal for reporting changes in that flag.  The item will
become invisible in situations according to the new hidden-when=''
attribute, which can be set to 'action-disabled' or 'action-missing'.

This new flag doesn't actually do anything yet, and none of the
consumers of GtkMenuTracker do anything with it (nor should they).  A
followup patch will address the issue.

https://bugzilla.gnome.org/show_bug.cgi?id=688421